Aircraft flight control systems are designed with extensive redundancy to ensure a low probability of failure. During recent years, however, several airplanes have experienced a major flight control system failure leaving engine thrust as the only mode of flight control. In some of these emergency situations, engine thrust was modulated by the pilot to maintain control only of the flightpath angle .gamma., but in other situations lateral control was also required. In the majority of the cases surveyed, crashes resulted and over 1,200 people died.
Thus, the challenge was to create a sufficient degree of thrust modulation control by the Flight Control Computer (FCC) to safely fly and land an airplane using an Instrument Landing System (ILS). Meeting this challenge was the objective of a program to develop a Propulsion-Controlled Aircraft (PCA) emergency backup system. The proposed PCA system required that the aircraft have at least two engines, one on each wing, and that the normal control surfaces not be locked in a hardover position which could exceed the moments that could be created from the controlled thrust of the engines.
The results of analytical studies and flight tests show that control can be obtained by autocontrol of the engine throttles through the FCC. However, making a safe runway landing is exceedingly difficult because of low phugoid damping and high pilot work load near the ground. The problem has been to not only improve emergency control performance but also reduce pilot work load using a PCA system. The end goal was to make landing as well as climbing, cruising at altitude, and descending a viable task by using the PCA system with minimal or no previous pilot training required.
In the recent past, a propulsion controlled system has been developed and disclosed in U.S. Pat. No. 5,330,131 by Frank W. Burcham et al. (incorporated herein by this reference) based upon a program modification of an otherwise normal FCC that, in the event of a failure of the normal flight control system of a multiengine airplane, substitutes normal pitch axis control with symmetric control of the engines through their FCC driven servos using pitch attitude and pitch rate sensed by gyros to provide the feedback signals necessary to track a pitch command signal. That patented flight control system requires modification not only of the FCC but also of each full-authority digital engine control (FADEC) computer for each engine. An objective of this invention is to provide a longitudinal emergency backup system for a PCA that requires only program modification of the FCC without any changes in the separate FADEC computers of the engines based upon tracking a flightpath angle command (.gamma.c) from a pilot thumbwheel, an Instrument Landing System (ILS), or the like, using a feedback signal from a flightpath angle sensor, or its reasonably equivalent altitude if there is not available on the aircraft a flightpath angle signal source aided by a feedback signal from a velocity sensor or, in a less complex method with some loss in performance, using only the sensed velocity signal.